Polycrystalline aluminum nitride substrates are useful in many different fields, such as construction materials in high-temperature machinery and in various electronic devices. They are typically recognized as providing valuable properties including high strength, oxidation resistance, thermal shock resistance, high thermal conductivity, low electrical conductivity, and resistance to corrosion by liquid metals. The properties of polycrystalline aluminum nitride substrates, however, can be altered by the presence of impurities, such as oxygen, carbon, and metals.
Polycrystalline aluminum nitride substrates typically have been prepared by high pressure, high temperature sintering processes. It has been recognized in the art, however, that sintering aids or binders are required to form useful substrates. For example, known methods have required the use of high oxygen content aluminum nitride powder (i.e., a low purity AlN powder) or have required the addition of sintering aids or binders, such as metal oxides, metal nitrides, and metal hydrides—e.g., aluminum oxide (Al2O3), yttrium oxide (Y2O3), calcium oxide (CaO), barium oxide (BaO), strontium oxide (SrO), lithium oxide (Li2O), lithium yttrium oxide (LiYO2), boron nitride (BN), calcium nitride (Ca3N2), strontium nitride (Sr3N2), barium nitride (Ba3N2), calcium hydride (CaH2), strontium hydride (SrH2), and barium hydride (BaH2).
Sintering aids typically are included in polycrystalline aluminum nitride substrates to achieve acceptable density at lower sintering temperatures by reducing the sintering activation energy or to increase density at higher temperature than would otherwise be possible. Binders typically are recognized as additives that can help hold powders together as they are pressed. In most cases, some content of the binder is burned away during the heating that leads up to the sintering process.
While sintering aids and binders can facilitate formation of polycrystalline structures, the formed structures can have reduced benefit. For example, the addition of oxides can adversely affect thermal conductivity, and uneven sintering can occur with the addition of many sintering aids. The use of sintering aids also can prevent formation of dense polycrystalline substrates having particularly desirable surface morphologies. Likewise, incomplete binder removal during heating can interfere with sintering and result in low final density. Thus, there remains a need in the art for further methods of preparing polycrystalline aluminum nitride materials and for the provision of such materials having particularly desirable surface morphologies.